In an ultrasonic diagnostic apparatus that transmits ultrasound to a subject and receives a reflected wave of the ultrasound to obtain an image, an ultrasound probe that transmits and receives the ultrasound between the ultrasound probe and the subject is used. It is known that, in the ultrasound probe, an acoustic lens that comes into contact with the subject, a transducer that mutually converts the ultrasound and an electric signal, and a backing layer that absorbs the ultrasound radiated on the rear face side of the transducer are provided.
As such a transducer, CMUT (Capacitive Micromachined Ultrasonic Transducers) is described in Non Patent Literature 1. The CMUT is formed by patterning a large number of CMUT cells (hereinafter referred to as capacitive vibration elements as appropriate.) on a semiconductor substrate using a lithography technique. The CMUT cells have a structure in which a recess is formed in an insulating layer formed on a semiconductor substrate, an opening of the recess is closed by a membrane to form a vacuum (or gas-filled) gap, and a pair of electrodes are provided to be opposed to each other on the front face of the membrane and the rear face of the insulating layer across the vacuum gap. The CMUT cells apply an electric signal having an ultrasonic frequency between the pair of electrodes to thereby vibrate the membrane and transmit the ultrasound to the inside of the subject. The CMUT cells receive, in the membrane, reflection echoes from the inside of the subject and convert displacement of the membrane into an electric signal as a change in capacitance between the pair of electrodes. The plurality of CMUT cells are separated from one another by a frame body composed of an insulating layer. One oscillator is formed by an aggregate of the plurality of CMUT cells having such a structure. A plurality of such oscillators are one-dimensionally or two-dimensionally arrayed on the same semiconductor substrate to form an ultrasound probe. The CMUT has advantages that, for example, a frequency band of usable ultrasound is wide and sensitivity is high compared with a transducer made of piezoelectric ceramic.
On the other hand, the CMUT can increase and decrease an electromechanical coupling factor by applying a direct-current bias between the electrodes and increasing and decreasing the direct-current bias voltage. However, in order to increase the sound pressure of the ultrasound, it is necessary to reduce the electromechanical coupling factor. Therefore, when it is attempted to obtain desired sound pressure, efficiency of conversion of the ultrasound into the electric signal is deteriorated. Therefore, in general, the CMUT has low conversion efficiency of the ultrasound compared with the transducer made of the piezoelectric ceramic. When the conversion efficiency is low, the reflection echoes from the subject are not converted into an electric signal and are transmitted through the semiconductor substrate to reach an interface of the backing layer and are reflected. As a result, a problem of multiple reflection in which the reflection echoes are repeatedly reflected between the subject and the interface of the backing layer occurs. In order to suppress such multiple reflection, Patent Literature 1 proposes to match acoustic impedances of the semiconductor substrate and the backing layer.